gender specific differences in activity and home range reflect morphological dimorphism in wolf ...

13 Pages·2005·0.37 MB·English

2005. The Journal of Arachnology 33:334–346
GENDER SPECIFIC DIFFERENCES IN ACTIVITY AND HOME
RANGE REFLECT MORPHOLOGICAL DIMORPHISM IN
WOLF SPIDERS (ARANEAE, LYCOSIDAE)
Volker W. Framenau1: Department of Zoology, The University of Melbourne,
Parkville, Victoria, 3010, Australia. E-mail: [email protected]
ABSTRACT. Sexual dimorphism of locomotory organs appears to be common in a variety of arthro-
pods, however, the underlying evolutionary mechanisms remain poorly understood and may be the con-
sequenceof naturalorsexualselection,oracombinationofboth.Ianalyzedtheactivitypatternofseven
cohorts of a wolf spider, Venatrix lapidosa, over four consecutive years. Males appear to be the more
active sex in search for a mate as they show temporarily higher activity prior to the periods of female
brood care. Morphometric data on leg length showed comparatively longer legs for males than females.
Allometriclegelongationinallfourlegsofmalesarisesonlyafterthefinalmoltsuggestingitssignificance
in reproductive behavior such as mate search. A comparative analysis of two Australasian wolf spider
generawithdifferentactivityprofileoffemales,Venatrix(sedentaryfemales)andArtoria(vagrantfemales)
provides further evidence that limb elongation in males mainly arises due to indirect male mate compe-
tition.
Keywords: Sexualdimorphism,locomotion,leglength,markandrecapture,minimumconvexpolygon
Sexual dimorphism is thought to have thussexualdimorphismcouldevolveinaspe-
evolved through sexual selection, ecological ciesthroughbothsexualandnaturalselection.
niche partitioning, differences in reproductive Therefore, it is often difficult to determine
roles or a combination of these factors (e.g., what mix of influences has resulted in sexual
Selander 1972; Hedrick & Temeles 1989; dimorphisminaparticularspecies(Hedrick&
Shine 1989; Reynolds & Harvey 1994; Fair- Temeles 1989).
bairn 1997). Sexual selection arises through The difficulty of identifying selective pres-
competition between members of one sex for sures is especially evident in the sexual di-
reproduction with the other sex (Andersson morphism of locomotory structures, like
1994). Ecological niche partitioning may re- wings or legs, which is a common phenome-
sultinsexualdimorphismifeachsexdevelops non in many arthropods (Montgomery 1910;
different structures as adaptations to different Thornhill & Alcock 1983). The evolution of
resources (Shine 1989; Walker & Rypstra gender specific differences in locomotory or-
2001). Different reproductive success primar- gans may be favored by both selection on
ily arises through a fecundity advantage of male mate searching behavior and natural se-
large body size in females and is particularly
lection on female movements in relation to
evidentininsectsandspidersinwhichacom-
foraging or oviposition. Therefore, sexual di-
mon finding is that, throughout a wide range
morphism of locomotory structures has gen-
of sizes, female fecundity varies directly with
erallynotbeenconsideredinstudiesofsexual
mass (e.g., Head 1995; Prenter et al. 1999).
selection (Darwin 1871; Andersson 1994).
Selection for early maturation of males (pro-
Gender specific differences in locomotory
tandry)mayalsofavorsmallermalebodysize
structures have usually been attributed to a
and thus result in sexual dimorphism(Bulmer
more active behavior of one sex, typically
1983; Gunnarsson & Johnsson 1990). These
males, in search for mates (Thornhill & Al-
explanations are not mutually exclusive and
cock 1983; Gasnier et al. 2002). Higher mo-
bility may increase encounter rates of males
1Current address: Department of Terrestrial In-
with females and therefore increase fertiliza-
vertebrates, Western Australian Museum, Locked
Bag 49, Welshpool DC, Western Australia 6986, tion success. However, gender specific elon-
Australia gation of limbs,even if undertheinfluenceof
334
FRAMENAU–ACTIVITYAND SEXUALDIMORPHISMIN WOLF SPIDERS 335
sexual selection, may not indicate an advan- inglifestrategiesthatrangefrompermanently
tage in locomotion. Male elongated legs are burrowing (e.g., Geolycosa or Lycosa s. str.),
important for direct male competition for ma- topermanentlyvagrantanimals(e.g.,Pardosa
tesinwaterstriders(Tseng&Rowe1999)and and Pirata; e.g., Dondale & Redner 1990).
megalopodine beetles (Eberhard & Marin These different lifestyles are reflected in me-
1996), in grasping females during mating in chanics of locomotion and activity response
mayflies or calanoid copepods (Peters & to variation in food supply (Ward & Hum-
Campbell 1991; Ohtsuka & Huys 2001), in phries 1981; Walker & Rypstra 2001). There-
newt courtship displays (Malmgren & Thol- fore, it is important to analyze sexual dimor-
lesson 2001) and to reduce the risk of sexual phism in locomotory organs in conjunction
cannibalism in some orb-web spiders (Elgar with data on the general activity pattern over
et al. 1990). In cursorial spiders, elongated an adult spider’s life span.
segments of legs, in particular the first pair, The goal of this study was to relate the ac-
have also been reported in combination with tivity profile of males and females of a cur-
ornamentations in species with visual court- sorial wolf spider, Venatrix lapidosa (McKay
shipdisplay(Kronestedt1990;Hebets&Uetz 1974), to gender specific differences in the
2000). Therefore, it is vital to correlate activ- morphology of their locomotory organs. The
ity and mobility patterns with sexual dimor- activity profile of these spiders was generated
phism of leg length to provide evidence of by conducting a fortnightly mark and recap-
sexual selection acting on locomotion itself. ture survey over a period of more than three
Sexual dimorphism in spiders has been years,coveringsevengenerationsofadultspi-
studied extensively, however, the evolution of ders.Thisallowedananalysisofboththevar-
sexual size dimorphism remains controversial iation of spider activity over their entire adult
(e.g., Elgar 1991; Vollrath & Parker 1992; life, and incorporated seasonal variation, thus
Head 1995; Hormiga et al. 1995). There are contrasting with all previous studies of wolf
two main explanations for patterns of sexual spiders that typically observed individuals for
size dimorphism in spiders (see Elgar 1998). only up to a day (Richter et al. 1971; Cady
Firstly, fecundity selection may favor larger 1984). I was not only interested in each in-
females (Prenter et al. 1997, 1998, 1999). Al- dividual’s activity (i.e. movement per unit
ternatively, Vollrath & Parker (1992) suggest time),but alsothespatialaspectofmovement
that sexual dimorphism may arise from dif- (homerange).Increasesofbothvariableshave
ferences in male and female lifestyles.Inspe- the potential to augment fertilization success
cies with sedentary females, an increase in of males by increasing their encounter rates
male mortalitythroughmatesearchingbehav- with females. However, these variables may
ior relaxes selection for large male body size not co-vary and higher activity may not nec-
and thus selection for protandry will favor essarilyincreasehomerangesize.Differential
smaller males. Ground living spiders are gen- spatial use by males and females, as inferred
erally less size dimorphic than web-building fromtheirhomerange,mayalsoinfluencethe
species, which has been explained by their operationalsexratio,therebyaffectingthepo-
differing reproductive and foraging strategies tential for male-male competition. Lastly, I
(Enders 1976; Prenter et al. 1999). There is analyzed locomotory structures of two Aus-
some evidence for sexual dimorphism in lo- tralasian genera of wolf spiders, Venatrix and
comotory structures in ground living spiders Artoria, with different activity profiles of fe-
(e.g., Gasnier et al. 2002). Montgomery males to determine if differences in behavior
(1910)reportedthatmaleshaverelativelylon- are reflected in leg length dimorphism across
ger legs than females, which he suggested is a higher taxonomic level.
aresultofthenomadicbehaviorofmalesafter
METHODS
attaining sexual maturity. This idea is sup-
ported by a number of short term studies on Study species.—Venatrix lapidosa is a
the locomotory activity of wolf spiders, in nocturnalwolfspiderinhabitingripariangrav-
which males were the more active sex (e.g., el banks in southeastern Australia (McKay
Hallander 1967; Richter et al. 1971; Cady 1974; Framenau &Vink2001).Itisavagrant
1984; Framenau et al. 1996a). However, wolf species, but brood caring females, and allspi-
spiders differ in activity profiles due to vary- ders during overwintering, dig excavations
336 THE JOURNALOF ARACHNOLOGY
Figure1.—Relativeleglength(residualsofleglengthoncephalothoraxwidth)(mean6s.e.)offemale
and male penultimate and adult Venatrix lapidosa. For statisticalanalysis (ANOVA)see Table 1.
under rocks that they line with a thin layer of jor catchments during a survey of riparian
silk (Framenau 1998, 2002a). Venatrix lapi- gravel banks in the Victorian Alps between
dosa is biennial, with juvenile development November 1999 and January 2000 (Framenau
requiring up to 16 months. Adult life span of et al. 2002). Leg length (sum of all segments
females may be up to eleven months and that measured dorsally) of all four pairs of legs
of males up to ten months (this study; also was determined under a stereomicroscope to
Framenau & Elgar 2005). However, the av- thenearest0.1mm.Carapacewidthwasmea-
erage life span of adults of both sexes does sured above the coxae of the second pair of
not generally exceed 6 months. The life cycle legs as an indicator of spider size (Hagstrum
of V. lapidosa in the Victorian Alps is char- 1971; Jakob et al. 1996). I included penulti-
acterizedbythematurationoftwodistinctco- mate spiders in the analysis to establish if an
horts within each year (Framenau & Elgar allometricincreaseinleglengthoccursduring
2005). In autumn maturing cohorts, most in- the last molt, potentially indicatingtheimpor-
dividualsmaturebetweenMarchandMay,en- tance of dimorphism for mature, sexually ac-
terwinterdiapause,reproduceonlyafterover- tivespiders.Youngerthanpenultimatespiders
wintering and die by December. In spring werenotusedasitwasimpossibletoestablish
maturing cohorts, spidersmolttomaturitybe- their sex. Gender specific differences in leg
tween November and January, reproduce im- length were analyzed using the residuals of a
mediatelyandmostspidersdiebyMay.Over- least squares regression using leg length on
lap between adult individuals of both cohorts cephalothoraxwidthoverbothsexesandadult
is minimal and generally limited to a low and penultimate spiders. This gives rise to
number of long-lived individuals that reach measures of relative leg length, which were
the maturation period of the followingcohort. independent of body size. Residuals between
Laboratory-reared males and females of dif- the sexes and adult and penultimate spiders
ferent cohorts readily mate (Cutler 2002) and were compared by two-way ANOVA.
this overlap permits gene flow between the Mark and Recapture.—The mark and re-
differentcohorts.Wintercoversalargeperiod capture study of V. lapidosa was conducted
of the adult life span of autumn maturing spi- on a gravel bank at the Avon River near Va-
ders, whereas adult individuals of the spring lencia Creek in Victoria, southeastern Austra-
maturingcohortsliveoversummer,socohorts lia(378489S,1468279E).Theclimateofthere-
were expected to differ considerably in their gion is moderate with mean daily maximum
activity profile. andminimumtemperaturesof20.08Cand8.0
Morphology of V. lapidosa.—I collected 8C, respectively. Annual rainfall averages594
adult (9 females, 15 males) and penultimate mm (Data from Maffra Forestry Office; Bu-
(15 females, 12 males)V.lapidosa fromava- reau of Meteorology, Melbourne). The gravel
riety of populations at ten rivers in seven ma- bank studied was borderedbytheAvonRiver
FRAMENAU–ACTIVITYAND SEXUALDIMORPHISMIN WOLF SPIDERS 337
adhesive (Supaglue Gely). Cephalothorax
width and body length were determined with
vernier callipers to the nearest 0.1 mm. When
returned,mostspiderseitherremainedwithout
any movement under the same rock, or found
shelter under the next available rock. Initial
disturbancewasthereforeconsideredminimal.
On subsequent encounters,only a spider’spo-
sition was recorded to avoid further distur-
bance.
Activity was determined using the average
dailydistance(‘velocity’inSamietz&Berger
1997), which is defined as the distance be-
tween two consecutive fixes divided by the
Figure 2.—Individual average daily distances days between both observations. Not all spi-
(meters per day; mean 6 s.e.) of female and male ders were recaptured every survey and aver-
Venatrix lapidosa from the autumn and springma- age daily distances significantly decreased
turingcohortsattheAvonRiver.Averagedailydis- with the time lapsed betweentwoconsecutive
tance moved based on two-week interfix intervals. fixes (two-, four-, six-, eight-weekly interfix
Todetermineindividualaveragedailydistances,all intervals; R2 5 0.072, P , 0.001, n 5 2,706).
average daily distances measured during the life
Thus, only average daily distances based on
span of one individual were averaged.
recaptureswithintwoweeksofapreviousone
were considered in the analysis. In addition,
onthesouthernsideandadensecoverofveg- these shorter intervals provided the most ac-
etation(wattle,Acaciatrilobata,willow,Salix curate picture of a spider’s movement. I com-
sp. and blackberry, Bromus sp.) on a steep paredindividualactivityofmalesandfemales
slope on the northern side, keepingspiderim- (‘individual average daily distance’) and au-
migration and emigration minimal. With the tumn and spring maturing cohorts by their
exception of a few Acacia and Salix shrubs, mean average daily distances over the whole
the gravel bank was bare ofvegetation.When observation period. To analyze seasonal vari-
the survey commenced in 1996, the surface ability of activity, average daily distances
area of the bank was 1,830 m2. In August were also determined for each month of the
1998, it diminished in size to 1,540 m2 due to year pooled over all individuals of each sex
a severe flood. butanalyzedseparatelyforautumnandspring
A 5 m 3 5 m grid was established on the maturingcohorts.Inthiscase,theaveragedai-
gravel bank using wooden pegs. One co-or- ly distance was obtained by analyzing cap-
dinate of this grid (‘Y’-value) was perpendic- tures within two consecutive months were as-
ular to the river and therefore expressed the signed to the month that contained most days
relativedistancefromthewater.Surveyswere of the interfix interval. I pooled monthly data
conductedfortnightly,from8November1996 over all years after establishing that therewas
to 2 May 2000. This observation period cov- no between year variation.
ered four spring maturing cohorts (1996– Home range.—I estimated home ranges
1999) and three autumn maturing cohorts using 100% minimum convex polygons
(1997–1999)(Table2).Allrockslargeenough (MCP;Mohr1947).Forlowcapturenumbers,
to provide shelter for spiders were overturned MCPs increase with each additional fix until
andreplaced.Eachsurveystartedrandomlyat a stable home range is reached. Regression
either end of the gravel bank. Each grid was analysis identified nine as the minimum num-
examined in a spiral from exterior to interior, berofcapturesfromwhichanincreaseinfixes
inordertopreventspidersfromleavingagrid did not result in a further, significant increase
while it was searched. Spider locations were in home range size (R2 5 0.008, P 5 0.397,
determinedtowithinanaccuracyof1m.New n 5 91). Increment analysis of home ranges
adult spiders in the population were individ- (Kenward & Hodder 1996) showed that nine
ually marked with a bee tag glued to their fixes provided an average of 90% of the full
cephalothorax using a cyane-acrylate based home range. This conformsto resultsofSam-
338 THE JOURNALOF ARACHNOLOGY
ietz & Berger (1997), who show that home with ANOVA assumptions were log-trans-
ranges (100% MCP) for insects appear to be formed,incaseofaveragedailydistances(log
stable from 10 captures. Two-week observa- 11)-transformed (Quinn & Keough 2002). If
tion periods guaranteed temporal indepen- normality of data could not be achieved, non-
dence of subsequent fixes, which is assumed parametric tests (Mann-Whitney U Test)were
if an animal can cross its home range within usedtocomparesexes.Measurementsaregiv-
thisperiod(White&Garrott1990).Asamea- en as mean 6 standard error (s.e.) unless oth-
surement of home range shape, I calculated erwise indicated. Voucher specimens of V.
the range span as the distance of the furthest lapidosa were deposited at the Museum Vic-
two points in a home range. Range centers toria, Melbourne, and the Western Australian
were calculated as the mean of the X- and Y- Museum, Perth.
values, corresponding to the established grid,
RESULTS
of all fixes in a home range. Only one range
in the spring maturing cohorts was based on MorphologyofV.lapidosa.—Thecarapace
morethaneightfixes(Table2).Therefore,be- width (6 s.e.) of adult female V. lapidosa
tween cohort analysis of home range size and (6.66 6 0.18 mm, n 5 9) was significantly
range span was not possible. larger than that of males (5.91 6 0.06 mm, n
Home range overlap was calculated for 5 15; separate t 5 4.019, d.f. 5 10.2, P 5
pairs of spiders that belonged to the same co- 0.002). The length of all legs was positively
hort and so could potentially meet. Two val- correlatedwithcephalothoraxwidthandresid-
ues of home range overlap could be deter- uals of these regressions yielded measures of
mined for each pair of spiders, i.e. how much relative leg length (Table 1). All legs were
ofthehomerangeofspiderAwasoverlapped comparativelylongerinmalesthaninfemales
by the range of spider B, and vice versa. I for adult and penultimate spiders (Table 1,
used the average of both values as the mea- Fig. 1). A significant interaction between age
surement of range overlap. and sex for all legs indicates a proportionally
Comparative morphology.—The taxono- higherelongation(allometricgrowth)formale
my of only two Australasian wolf spider gen- legs during their last molt compared with fe-
era, Venatrix Roewer (Framenau & Vink males (Fig. 1).
2001; 23 species) and Artoria Thorell (Fra- Mark and recapture survey.—A total of
menau2002b;11species)isknownsufficient- 741 males and 712 females were individually
lytoallowinterspecificcomparativeanalyses. markedoveraperiodof3.5years,yieldingan
Both genera differ considerably in their mo- overall even sex ratio (x2 5 0.802, P 5 0.37)
bility pattern, as most femalesofVenatrixdig (Table 2). However, recapture rates, i.e. how
permanentburrowsorconstructtemporaryex- often individual spiders were caught, differed
cavations during brood care, whereas females significantly for males and females (Mann-
of Artoria are vagrant throughout their life Whitney U 5 314226.5, P 5 0.008) due to a
(Framenau 2002b; also pers. obs). Least higher number of males encountered only
squares regression of leg length on cephalo- once. The total number of fixes analyzed was
thorax width for all pairs of legs over all spe- 4,963 yielding a detailed life cycle profile for
cies derived from the primary taxonomic lit- each cohort in each year(seeFramenau&El-
erature of both genera (Framenau & Vink gar 2005).
2001; Framenau2002b)providedmeasuresof Activity.—Meanaveragedailydistancesof
relative leg length (residuals) compared to a individualspiders,basedontwo-weekinterfix
‘typical’ lycosid. To test for sexual dimor- intervals, were significantly higher for males
phism within both genera, these residuals thanfemales,andhigherforindividualsofthe
were compared between the sexes using two- spring mating cohorts than of the autumnma-
sample t-tests. turing cohorts (two-way ANOVA; sex: F
1,699
Statistical analysis.—Home range and ac- 5 6.045, P 5 0.014; cohort: F 5 4.816, P
1,699
tivity parameters were calculated using the 5 0.029; interaction: F 5 0,384, P 5
1,699
software package ‘RANGES V’ (Kenward & 0.536) (Fig. 2).
Hodder 1996). Subsequent statistical analyses Monthly average daily distances in the au-
were performed with ‘SYSTAT Version 99 tumn cohort showed no significant difference
(SPSS Corp. 1998). Data that did not comply between sexes (two-way ANOVA; F 5
1,1186
FRAMENAU–ACTIVITYAND SEXUALDIMORPHISMIN WOLF SPIDERS 339
Figure3.—Monthlyaveragedailydistances(metersperday;mean6s.e.)offemaleandmaleVenatrix
lapidosa from the autumn and spring maturing cohorts during the survey at the Avon River. The month
on the far left in each graph represents the maturation of each cohort. Average daily distance moved is
based on two-week interfix intervals. To determine monthly average daily distances, all average daily
distances within a month were averaged over all individuals. Asterisk (*) indicates significant difference
between sexes.
0.087, P 5 0.769), however, there were sig- there was no differences between months
nificantdifferencesbetweenmonths(F 5 (F 5 2.188, P 5 0.055; interaction: F
10,1186 5,389 5,389
7.908,P,0.001;interaction:F 51.055, 5 0.892, P 5 0.486; May–October excluded
10,1186
P 5 0.394; January excluded due to missing duetomissingvarianceinsexormonths;Fig.
variation between sexes; Fig. 3). In the spring 3). However, a within months comparison of
cohort, there was also no overall gender spe- averagedailydistancesbetweenmalesandfe-
cific difference in monthly average daily dis- malesrevealedsignificantlyhighermaleactiv-
tances(two-wayANOVA;F 51.514,P5 ity in August (pooled t 5 3.048, d.f. 5 28, P
1,389
0.219) and, in contrast to the autumn cohort, 5 0.005) and September (pooled t 5 2.199,
Table 1.—Comparison of relative leg length (residuals of leg length on carapace width) betweenmale
andfemaleandadultandpenultimateVenatrixlapidosa.Regressionofleglengthoncarapacewidth:Leg
1: R2 5 0.543, slope 5 2.978, P , 0.001, n 5 51; leg 2: R2 5 0.608, slope 5 2.972, P , 0.001, n 5
51; leg 3: R2 5 0.689, slope 5 2.909, P , 0.001, n 5 51; leg 4: R2 5 0.672, slope53.457, P,0.001,
n 5 51. Given are the F -values and significance level (*P , 0.05, **P , 0.01, ***P , 0.001) of a
1,47
two-way ANOVA.
Factor Leg 1 Leg 2 Leg 3 Leg 4
Sex 60.918*** 55.644*** 35.171*** 26.328***
Age 74.322*** 76.209*** 39.534*** 39.378***
Interaction: Sex * Age 22.746*** 18.380*** 13.416** 7.923**
340 THE JOURNALOF ARACHNOLOGY
n 5 50; pooled t 5 0.548, d.f. 5 89; P 5
0.585). Home range centers did not show a
significant difference between sexes alongthe
river (X-coordinate; pooled t 5 1.732, d.f. 5
89; P 5 0.087), but the relative distancefrom
theriver(Y-value)wassignificantlyhigherfor
females (10.1 6 0.2, n 5 49) than males (9.3
6 0.2, n 5 42; pooled t 5 3.156, d.f. 5 89;
P 5 0.002). In addition, females carrying an
eggsac were found significantly further away
fromthewater(Y-coordinate6s.e;9.863.3,
n 5 124) than females not caring for brood
Figure 4.—Intra- and intersexual home range (9.0 6 3.0, n 5 1,179; pooled t 5 2.372, d.f.
overlap (mean 6 s.e.) in Venatrix lapidosa within 5 1301; P 5 0.018). Overall, range overlap
the 1997 autumn maturing cohort. No other cohort was high (. 50%, Fig. 4), but it differed sig-
provided a sufficient number of home range esti- nificantly between sexes, with male-male
mates to compare between and within sexes (see
overlap highest and female-female overlap
Table 1). lowest(ANOVA;F 519.315,P,0.001)
2,3743
(Fig. 4).
d.f. 5 221, P 5 0.029) for the autumn cohort Comparative morphology.—There was a
and in January (pooled t 5 2.673, d.f. 5 120, positive correlation between cephalothorax
P 5 0.026) for the spring cohort (Fig. 3). width and leg length in both Venatrix and Ar-
toria and measures ofrelativeleglengthwere
Home range.—Due to the major flood in
obtained from the residuals of the respective
1998, home range overlap analysis was re-
regressions (Table 3, Fig. 5). Males had com-
stricted to the autumn 1997 cohort when
paratively longer legs than females within the
males and females were caught in sufficient
genus Venatrix, but there was no gender spe-
numbers (minimum of nine fixes) to allow a
cific difference in the relative leg length in
comparison between the sexes (Table 2).
Artoria (Table 3, Fig. 5).
Therefore, a comparison between cohortswas
not possible. Home range estimates (100%
MCP 6 s.e.) did not differ significantly be- DISCUSSION
tween males (302 6 16 m2, n 5 42) and fe- There was a considerable difference in the
males (311 6 17 m2, n 5 49; pooled t 5 activity and mobility pattern of male and fe-
0.375, d.f. 5 89; P 5 0.709). Home range male V. lapidosa, which corresponded to a
span also did not differ between males (55.4 pronounced sexual dimorphism in the length
6 2.1 m, n 5 42) and females (53.7 6 2.2m, of their legs. The evolution of longer legs in
Table 2.—Capture statistics of the mark and recapture survey of Venatrix lapidosa at the Avon River,
distinguished by cohorts. Average daily distance based on two-weekly interfix intervals only (see text).
§Considered in analysis of average daily distances;‡Considered in home range analysis.
Cohort
Spring Autumn Spring Autumn Spring Autumn Spring
1996 1997 1997 1998 1998 1999 1999
Males:
Total number marked 115 179 80 168 86 84 29
Recaptured at least once§ 58 157 35 121 43 67 9
Minimum of nine captures‡ 0 38 0 2 0 2 0
Females:
Total number marked 127 176 105 140 77 66 21
Recaptured at least once§ 88 158 72 96 48 56 7
Minimum of nine captures‡ 1 48 0 0 0 0 0
FRAMENAU–ACTIVITYAND SEXUALDIMORPHISMIN WOLF SPIDERS 341
Figure5.—Relativeleglength(residualsofleglengthoncephalothoraxwidth)(mean6s.e.)offemale
and malespecies ofVenatrixand Artoria.Forstatisticalanalysisbetweensexesineachgenus(t-test)see
Table 3.
males may be the result of an increased like- the autumn maturing cohort, males emerge
lihood to encounter more stationary females earlierfromdiapauseandaremoreactivethan
assuming a higher energy efficiency or speed females two months prior to female egg pro-
as a result of leg elongation. The lack of sex- duction, suggesting that males are searching
ual dimorphism in leg length in juvenile V. for mates. Higher male activity in the spring
lapidosa and the genus Artoria (in which fe- mating cohort cannot be as easily explained
males are vagrant) supports this argument. in terms of mate searching, as male activity
Activity.—Venatrix lapidosa is a compar- was particularly high in January, when fe-
atively immobile spider. Similar low activity males had already commenced egg produc-
occurs in other cursorial spiders inhabiting tion. Since higher male activity is observed
terrestrial-aquatic ecotones (Framenau et al. for only a few months, females appear to
1996a; Kreiter & Wise 2001). Limited mobil- movemorethansuggestedbypreviousstudies
ity of riparian species may be a result of their onwolfspiders(Richteretal.1971;Hallander
fragmented habitat consisting of generally 1967). Initial female activity may behighdue
small isolated gravel banks. In addition, high to increased foraging effort to meet energetic
preyavailabilitynearthewateredgemayren- requirements for egg production (Kreiter &
der it unnecessary to move (Greenstone Wise 2001). Movement in female V. lapidosa
1983). As expected for poikilothermic ani- may also be induced by the apparent prefer-
mals, activity between cohorts differed, most ence of females to oviposit some distance
likely reflecting seasonal patterns. Activity from the water. Activity may subsequently
was lower for individuals of the autumn mat- drop, as females becomesedentarytocarefor
ing cohort, due to a drop in movement over their brood (Hackman 1957; Hallander 1967;
winter. Individuals of the spring mating co- Framenau et al. 1996a, b; Nyffeler 2000). An
horts, although more active than the autumn unusually low proportion of ovipositing fe-
mating cohorts, showed no significant differ- males in V. lapidosa (Framenau & Elgar
ence in activity between months. These indi- 2005) compared to other lycosids (e.g., Fra-
viduals are adults mainly in summer. Temper- menau 1996a; Humphreys 1976) suggests a
ature dependent movement patterns have also comparatively low number of stationary,
been reported in other wolf spiders, such as broodcaring females and may partly explain
Pardosaamentata(Clerck1757)(Ford1978). whydifferencesinactivitybetweenmalesand
Activity patterns of males and females are females is limited.
similar within both cohorts, with males the Home range.—Despite the temporary in-
more active sex. A variety of studies on wolf crease in male activity, home range size did
spiders have shown that an increase in male not differ between males and females in V.
activity reflects mate searching (e.g., Hallan- lapidosa. The movements of V. lapidosa may
der 1967; Framenau et al. 1996a). In V. lapi- have been restricted by the size of the study
dosa, significantly higher male activity ap- site itself, but average home range size and
pears to be temporary, emerging about three range span for both sexes were considerably
months after maturation (delayed in the au- smaller than the surface and length of the in-
tumn maturing cohort by winter diapause). In vestigated gravel bank.
342 THE JOURNALOF ARACHNOLOGY
Table 3.—Comparisonofrelativeleglength(residualsofleglengthoncarapacewidth)betweenmales
and females in species of the genera Venatrix and Artoria. Regression of leg length on carapace width:
Leg 1: R2 5 0.899, slope 5 3.292, P , 0.001, n 5 51; leg 2: R2 5 0.912, slope 5 3.037, P , 0.001, n
5 58; leg 3: R2 5 0.903, slope 5 2.772, P , 0.001, n 5 58; leg 4: R2 5 0.892, slope 5 3.512, P ,
0.001, n 5 56. Given are the t-values (pooled variance) with significance level (n.s. non significant; *P
, 0.05, **P , 0.01, ***P , 0.001) and degrees of freedom (d.f.).
Factor Leg 1 Leg 2 Leg 3 Leg 4
Venatrix
Sex 4.140*** 3.056** 3.182** 2.039*
d.f. 33 34 34 34
Artoria
Sex 1.832 n.s. 1.529 n.s. 1.642 n.s. 0.770 n.s.
d.f. 19 20 20 18
Although home range size was similar be- been reported in wolf spiders (Aspey 1977a,
tween the sexes, there was a significant dif- b; Ferna´ndez-Montraveta & Ortega 1991).
ferenceinthedistributionofrangecentersbe- The effect of different habitat requirements,
tween females and males. Female range i.e. the search of a favorable location for
centerswere,onaverage,locatedfurtheraway brood care, appears to be stronger than male-
from the water as ovipositing females retreat female attraction or intrasexual aggression.
from the border of the gravel bank. Females Sexualdimorphism.—Venatrixlapidosais
may not tolerate a high degree of soil mois- sexually dimorphic. Females are generally
ture, or they may look for more protected ar- larger than males, but males have compara-
eas from varying water levels, before exca- tivelylongerlegs.ThesexratioofV.lapidosa
vation of brood chambers. Site specificity in was not biased in the autumn mating cohort,
relation to abiotic factors occurs frequentlyin and limited to later months in the spring co-
lycosids that build permanent burrows (e.g., horts when many females werealreadycaring
Humphreys 1976; Milasowszky & Zulka fortheirbrood(Framenau&Elgar2005).Fur-
1998).FemalesofA.cinerea(Fabricius1777) ther, higher home range overlap between
andTrochosaruricola(DeGeer1778),twoly- males suggests greater rather than less oppor-
cosids inhabiting shore habitats, also move tunity for male-male competition. Therefore,
away from the water prior to brood care mydataarenotconsistentwiththeunderlying
(Hackman 1957; Framenau et al.1996b).Dif- assumptions of the model developed by Voll-
ferential microhabitat preferences can have a rath & Parker (1992) that relates sexual size
strong influence on the activity and distribu- dimorphism in spiders to reduced male-male
tion patterns of individuals. In wolf spiders, competition due to an increase in mortality
intraspecific habitat preferences not only dif- caused by mate search. Sexual dimorphismin
fer betweenfemaleswithandwithouteggsacs V. lapidosa most likely evolved through a fe-
(Edgar 1971; Hallander 1967; Greenstone cundity advantage for larger females (Prenter
1983), but also between sexes (Cady 1984) et al. 1997, 1998, 1999); clutch size increases
and adults and juveniles (Edgar 1969, 1971; with body size in many wolf spider species
Kronk & Riechert 1979). (Marshall & Gittleman 1994; Simpson 1995).
The utilization of areas further away from While increased female fecundity may ex-
the water, together with equal home range plain size differences between males and fe-
size, may explain the lower female-female males, sexual selection through indirectmale-
rangeoverlapcomparedtomales.Inwolfspi- male competition may explain the
ders,malesandfemalesdonotencountereach comparatively longer legs of males. Allome-
otherhaphazardly.Malesfollowsilkdraglines tric growth leading to relatively longer legs
laid by receptive females which contain sex- only takes place in males and mainly during
attracting pheromones (Hedgekar & Dondale the final molt supporting an evolutionary hy-
1969; Tietjen & Rovner 1982). In addition, pothesisoflegelongationinmalesratherthan
strong agonistic behavior within sexes has leg shorting in females due to burrowing be-
FRAMENAU–ACTIVITYAND SEXUALDIMORPHISMIN WOLF SPIDERS 343
havior. The production of longer legs may be side of the cymbium of the pedipalps (Tietjen
ontogenetically costly and thus would be off- an Rovner 1982).
set by energetically more efficient movement. The comparison in the patternofleg-length
There are no experimental or comparative dimorphism in Venatrix and Artoria provide
data of increased movement efficiency with further evidence that male mate-searchingbe-
longer legs in arthropods (J. Shultz pers. havior favors relatively longer legs in males.
Comm.) and the relationship between leg di- Although males in Artoria also tend to have
mensions (length and thickness) and metabol- longerlegsthanfemales,thisdifferenceisnot
ic rate are complex and also entail the mass significantwithinthegenusandisfarlesspro-
of the spider. Simple lever mechanicspredicts nounced than in Venatrix. It appears unlikely
that if the length of the output lever arm in- that leg length dimorphism arises through
creases, the velocity and excursion at the end shortening of female legs due to burrowing
of the lever will increase (and thus speed and behavior,asallometricgrowthoccursbetween
distance moved per stride), but that the force penultimate and adult spiders. In addition,
the lever will exert will decrease (Manton thereisnoevidencethatfemaleVenatrixhave
comparatively shorter legs than vagrant Arto-
1977; Alexander 1982; Hildebrand & Goslow
ria.
2003).Malescancompensatethelossofforce
This study provides evidence that longer
by reducing their own mass which, in turn,
legs in male wolf spiders are mainly caused
augments selection for smaller males, provid-
by sexual selection through indirect competi-
ing a novel aspect in the explanation for sex-
tion with increased male activity in searching
ual size dimorphism in vagrant spiders. Over-
for a mate. To further elucidate the selective
all, longer-legged, smaller males are able to
forces responsible for the elongation of male
searchfasterandmoreextensivelyforfemales
legs, future work should focus on three ques-
and potentially increase their encounter rates
tions.Firstly, it isimportanttoexperimentally
with females. This advantage would be fa-
confirm the assumption that longer legs are
vored by sexual selection if it provided males
more energy efficient in spider movement.
with a competitive edge in terms of fertiliza-
Secondly, evidence that higher male activity
tion success.
will ultimatelyleadtohigherfertilizationsuc-
A limb elongation due to more efficient lo-
cessisrequired.Thisisstronglydependenton
comotion is also supported by the fact thatall
the speciesmatingsysteminquestion,andre-
four legs show the same allometric pattern
quires an understanding of multiple mating
whichwasnotrequiredifthedifferenceinleg
and sperm priority patterns of wolf spiders
length between sexes arose through sexual
(Austad 1984; Elgar 1998). Although V. lap-
cannibalism (Elgar et al. 1990), male-male
idosa has been reported to mate multiply in-
combats (Tseng & Rowe 1999), or in combi-
creasing the chance of male mate competition
nation with leg ornamentation in used in
(Cutler 2002), there is no information on
courtship display (Kronestedt 1990; Hebets&
sperm priority patterns in this and other Ly-
Uetz 2000). In addition, sexual cannibalism
cosidae. Lastly, comparative studies in leg
and male-male combats are extremely rare in
lengthdimorphismincomparisonwiththelife
wolf spiders (Aspey 1977a). Alternatively,
time activity patterns of cursorial spidersona
differentforagingbehaviorbetweenmalesand broader taxonomic base may help us under-
females could provide an explanation of sex- stand to what extent sexual dimorphism of
ualdimorphismbasedondifferentlocomotory limbs are under natural or sexual selection.
patterns (Givens 1978). However, due to low-
er metabolic requirements male wolf spiders ACKNOWLEDGMENTS
attack considerably fewer prey than females I am grateful to Melissa Thomas, Mark El-
(Walker & Rypstra 2001). Longer legs may gar, Douglas Morse, and Mark Harvey for in-
also provide a sensory advantage due to an valuable comments on earlier drafts of this
increased radius to mount olfactory chemo- manuscript. Jeff Shultz and Ken Prestwich
receptors or trichobothria. In wolf spiders, ol- provided important information on arthropod
faction plays some role in mate search, how- locomotion and energetics. Melissa Thomas,
ever, the main senses used by males to follow Fleur de Crespigny, Shar Ramamurthy, Eliz-
traillinesoffemalesaresituatedonthedorsal abeth Dalgleish, Romke Kats, Karen Blaak-